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Geology of the Ringwood district — brief explanation of the geological map Sheet 314 Ringwood

C M Barton, P M Hopson, A J Newell and K R Royse

Bibliographic reference: Barton, C M, Hopson, P M, Newell, A J, and Royse, K R. 2003. Geology of the Ringwood district—a brief explanation of the geological map. Sheet Explanation of the British Geological Survey. 1:50 000 Sheet 314 Ringwood (England and Wales).

Keyworth, Nottingham: British Geological Survey, 2003.

© NERC 2003 All rights reserved

Copyright in materials derived from the British Geological Survey's work is owned by the Natural Environment Research Council (NERC) and/or the authority that commissioned the work. You may not copy or adapt this publication without first obtaining NERC permission. Contact the BGS Intellectual Property Rights Manager, British Geological Survey, Keyworth. You may quote extracts of a reasonable length without prior permission, provided a full acknowledgement is given of the source of the extract.

(Front cover) Stephens Castle, Verwood [SU 0917 0967] (Photograph P M Hopson; GS 1248). One of numerous exposures of the Poole Formation showing interbedded laminated silty clay and fine- to medium-grained sand. The silty clay beds are lenticular and generally comprise 'ball clay' or kaolinite-rich clay.

(Rear cover)

(Geological succession) Summary of the geological succession at outcrop within the district.

Notes

The word 'district' refers to the area of Sheet 314 Ringwood. National grid references are given in square brackets. Most of the district lies within the 100 km square SU; the westernmost strip lies within the square ST. Borehole records referred to in the text are prefixed by the code of the National Grid 25 km² area on which the site falls, for example SU11SE, followed by its registration number in the BGS National Geosciences Records Centre.

Acknowledgements

This authors acknowledge the assistance provided by staff of the County Council at Dorchester. Macrofossil determinations during this resurvey were carried out by M A Woods. Seismic interpretation of the district and depth-converted seismic maps were done by M S Brook and C J Vincent. Lithological data from shallow seismic shot holes in the New Forest were recorded by R A Edwards and E C Freshney in 1983 to 1986. Landowners, tenants and quarry companies are thanked for permitting access to their lands. Figures were produced by P Lappage. This Sheet Explanation was edited by A A Jackson; figures were produced by R J Demaine, P Lappage and G Tuggey.

The grid, where it is used on figures, is the National Grid taken from Ordnance Survey mapping. © Crown copyright reserved Ordnance Survey. Survey licence number GD272191/2003.

Geology of the Ringwood district (summary from rear cover)

The Ringwood district is known for its unspoilt scenery and contrasting landscapes, and for the habitats associated with the south-flowing Hamsphire Avon. West of the river, the chalkland of Cranborne Chase forms rolling arable and grass-covered downs with sheltered streams and dry valleys. East of the river, sandy heathland of the New Forest forms high ground with numer- ous open plateaux. The river floodplain is largely devoted to pasture.

The concealed strata are known from seismic data and deep boreholes at Cranborne and Fordingbridge. They show that throughout most of the Triassic and Jurassic, the area was part of a structural high and these successions are relatively thin. The oldest exposed strata are in the north-west of the district where faulted and gently folded glauconitic sandstone of the Upper Greensand occur above the northern edge of the structural high. The overlying Chalk Group crops out across the entire west of the district and comprises a full sequence of largely unfaulted, chalk, dipping gently towards the south-east. All the important chalk sections occur in small, mostly disused pits. The upper part of the Chalk Group (Culver and Portsdown Chalk formations) thins towards the north of the district.

The upper part of the Cretaceous and the lower Paleocene sequence is cut out by a major uncon- formity. A sequence of siliciclastic sand and clay overlies the Chalk and thickens to a maximum of 300 m in the south-east of the district. The Palaeogene fill comprises eleven formations within five groups, and much of the sequence is laterally variable and concealed beneath drift. Stratigraphical divisions derived from adjacent ground are mapped across the district, and surface correlation is supported with borehole geophysical logs. The correlation of individual sand bodies, particularly in boreholes, is not straightforward, and formational boundaries can be difficult to map.

Chapter 1 Introduction

This Sheet Explanation provides a summary of the district covered by the geological 1:50 000 Series Sheet 314 Ringwood, which lies within the counties of Dorset, Hampshire and Wiltshire; the area is administered by the County Councils and the respective District Councils of East Dorset, New Forest and Salisbury. The small market town of Ringwood (population 13 000) is situated on the left bank of the River Avon (Hampshire), and other urban areas include Fordingbridge, Downton, Verwood, and Cranborne. Thus most of the district is rural, comprising the Chalk Downs of Dorset that include Cranborne Chase in the west and part of the New Forest National Park in the east. The rolling hills of Cranborne Chase are typified by thin soils developed on chalk bedrock. To the south and east beyond the low Palaeogene escarpment are flat-topped gravelly plateaux of heath and woodland of the New Forest. The River Avon flows southward across the district.

Structurally, the region is part of the Wessex Basin (Chadwick, 1986; Penn et al., 1987) that extended over much of southern England to the south of the London Platform and Mendip High, during Permian and Mesozoic times (Figure 1). Within this basin, the Ringwood district lay on the Cranborne– Fordingbridge High (also known as the Hampshire–Dieppe High), bounded by the Mere Fault with the Pewsey–Weald Basin to the north, and the Cranborne Fault with the Dorset Basin to the south. These faults are normal growth faults created by reactivation of deep-seated low-angle, southward dipping, thrusts in the underlying Palaeozoic strata. The Palaeozoic basement rocks were strongly deformed during the Variscan orogeny, a period of mountain building that culminated at the end of the Carboniferous. Beneath the Wessex Basin, the basement rocks are only weakly metamorphosed.

The oldest rocks, proved in the Cranborne Borehole, are attributed to the Old Red Sandstone red-bed facies of Devonian age. To the north and south of the structural high, rocks in the Netherhampton and Farley South boreholes have been attributed to the Carboniferous. A long period of erosion followed the Variscan orogeny so that a major unconformity marks the base of the Permo-Triassic sequence.

In Permian times, subsidence associated with periods of tectonic extension began to effect southern England initiating the development of fault-bounded, smaller basins within the Wessex Basin. The Dorset Basin that lies just to the south of the district contains a thin sequence of continental desert red-beds attributed to the Permian. In the Ringwood district red sandstone, calcareous siltstone and mudstone are attributed to the Triassic Sherwood Sandstone Group and Mercian Mudstone Group. In late Triassic (Rhaetian) times, the sea began to flood the Wessex Basin, and the Penarth Group marks this transgression.

The area of marine deposition increased gradually throughout the region during the Jurassic, but deposition was interrupted from time to time by episodes of erosion on the structural highs and towards the basin margins. Changes in thickness of strata across the basin indicate that there were major periods of active faulting during both Early and Late Jurassic times, and during the Early Cretaceous. Syndepositional movements on faults within the Wessex Basin resulted in thicker sequences of sediments in the basins (the downthrow side of the bounding faults) while on the highs the sequences are attenuated and some parts are absent. To the north about 1200 m of strata are present in the Pewsey–Weald Basin, and to the south more than 2000 m are present in the Dorset Basin. In the Ringwood district, thinner sequences were deposited on the Cranborne/Fordingbridge High, and a major attenuation is apparent in the Pliensbachian/Toarcian part of the Lias Group. With the exception of the extreme north-east of the district much of the Kimmeridge Clay Formation and all of the Portland, Purbeck, Wealden and Lower Greensand groups are absent. This is attributed to erosion associated with a marine regression that occurred during the Late Jurassic and Early Cretaceous.

Sedimentation was renewed over the Cranborne–Fordingbridge High in Albian times. The Gault and subsequently the Upper Greensand and Chalk Group were deposited across the whole district. Regional subsidence with intermittent reactivation of the major faults occurred during the Late Cretaceous, probably as a result of rapid sea-floor spreading in the North Atlantic. A large increase in depositional area, a relatively high sea level and the absence of a supply of land-derived sediment allowed the deposition of a pure carbonate Chalk sequence (Hamblin et al., 1992). In Cenomanian times, emergent landmasses were present in south-west England, Wales, Scotland and Northern Ireland and farther afield in Brittany and elsewhere. These land areas retreated further during the Campanian when the highest relative sea levels are recorded. Southern Britain, at this time, lay approximately 10° farther south than at present. Chalk accumulated on the outer shelf of an epicontinental subtropical sea of normal salinity and with little terrigenous input.

A drop in global sea level at the end of the Cretaceous, possibly coupled with regional uplift, resulted in erosion of the highest part of the Chalk and the development of a widespread pre-Cainozoic unconformity. Much of the Lower and Middle Paleocene sequence seen elsewhere is missing, as this region formed part of the land area that separated the Paris Basin and North Sea Basin at that time. The strata present in the district are assigned to the uppermost Paleocene (Thanetian) and Eocene. In a warm climatic regime, braided rivers deposited sand across this otherwise extensive swampy, clay lowland. After a short hiatus, the sea transgressed from the north, and the district formed part of a broad embayment that included the present London, Hampshire, Belgium and Paris basins. A further transgression in Eocene to Oligocene times resulted in deposition of essentially marine sediments across the whole of southern England, including this district. In 'mid-Tertiary' times the onset of the 'Alpine' compressive tectonic regime reversed the sense of movement on the major bounding faults of the Wessex Basin and resulted in inversion of the smaller basins and highs within it. Uplift is estimated to be as much as 1500 m (Simpson et al., 1989). Subsequent Neogene erosion has unroofed these inverted basins and together with remoulding during the Quaternary has resulted in the present-day landscape.

A synthesis of the geology of the district and surrounding region is given in Melville and Freshney (1982).

Chapter 2 Geological description

Concealed geology

The stratigraphy of the concealed strata beneath the district is known from hydrocarbon exploration data, principally the deep boreholes at Cranborne, Woodlands and Fordingbridge and from extensive seismic reflection profiles. A synopsis of these and adjacent boreholes is given in (Figure 2).

Devonian rocks underlie most of the district (Smith, 1985). In the Cranborne Borehole, they consist of pale grey and green siltstone and 'shale', interbedded with thin beds of mudstone and sandstone that is generally red or pink in colour. The beds are generally micaceous at the top and base of the sequence, and are dolomitic in places. Calcite veining is common, and the presence of mica and chlorite indicate low-grade metamorphism. Dip varies between 20° and 35°.

Beds attributed to the Carboniferous were proved in boreholes just to the north of the district at Netherhampton and Farley South (Figure 2); they consist of interbedded limestone, dolomite and chert. Permian strata are thought to rest with marked unconformity on the Devonian strata in the south and west of the district (Sellwood and Scott, 1986; Sellwood et al., 1986). In the Hurn Borehole, the strata consist of red mudstone overlain by coarse-grained conglomerate.

Triassic red beds and evaporite deposits are largely restricted to the south of the Wessex Basin and were proved in the Cranborne Borehole (Figure 2). Interpretation of seismic data shows that the beds thin northwards as they onlap onto the Cranborne–Fordingbridge High. Red and red-brown, fine- to coarse-grained sandstone with sparsely distributed, thin, grey and red mudstone and siltstone beds have been attributed to the Sherwood Sandstone Group. These are overlain by red and grey, calcareous mudstone and siltstone, with discrete beds of evaporitic minerals chiefly anhydrite and rare, thin dolomitic limestones of the Mercia Mudstone Group.

The top of the Triassic lies at about 1200 m below OD beneath most of the Ringwood district, although the reflector that marks the top is deeper along a narrow belt in the north of the district and along the southern boundary. The Penarth Group, overlies the Mercia Mudstone Group, and indicates a return to marine conditions. This comprises black fissile mudstone and siltstone overlain by white limestone.

Lower and Middle Jurassic strata are present beneath the district (Figure 2); the beds dip gently towards the east and are estimated to be between 630 and 778 m thick. Typically, the reflector marking the top Inferior Oolite occurs at about 800 to 1000 m below OD, the Cornbrash at 650 to 850 m and the Corallian at 350 to 700 m. The deposits comprise shallow marine mudstone, sandstone and limestone. The Kimmeridge Clay is less than 50 m thick in the Cranborne Borehole, about 160 m thick beneath the north-east of the district compared with more than 500 m on the Dorset coast. The base of the unconformity ranges from about 50 m below OD in the north-west of the district to about 700 m in the south-east. The Kimmeridge Clay Formation (along with the Lias and Oxford Clay) is generally considered as the potential hydrocarbon source rock south of the Variscan front. Production is mainly from structural traps within the Bridport Sand and Great Oolite or within the sandstones of the Permo-Trias sequences to the south (i.e. Wych Formation near Poole Harbour). Concealed Cretaceous strata include the Lower Greensand and Gault, proved in boreholes and known from surface exposure in the Shaftesbury district to the west (Bristow et al., 1995). The Lower Greensand Formation comprises fine-grained glauconitic sand and clayey sand; some iron cemented beds occur near the base. The Gault Formation extends fairly uniformly across the Wessex Basin, comprising 37 to 41 m of sandy, micaceous, patchily glauconitic mudstone with sand and poorly cemented sandstone.

Cretaceous

Upper Greensand Formation (UGS)

The Upper Greensand is a sequence of bedded siltstone and sandstone. The siltstone is pale yellow brown, pale grey and greenish grey in colour and bioturbated; the sandstone is very fine grained silty and weakly cemented. At surface, it gives rise to a characteristic blocky angular brash that weathers to a greenish hue from which the formation derives its name.

The upper 10 m of the formation crops out along the base of the primary Chalk scarp between Berwick St John [ST 9500 2233] and Alvediston [ST 9765 2342] in the north-west of the district. Here the full sequence is approximately 60 m thick. The formation thins southward, and the upper part of the succession is absent in the south-west of the district, where either it was never deposited, or has been subsequently eroded from the crest of a high, known as the Mid-Dorset Swell (Drummond, 1970; Bristow et al., 1995). The Upper Greensand is divided into three members: in ascending order these are the Cann Sand (fine-grained sand), Shaftesbury Sandstone (glauconitic fine- to medium-grained sandstone), and Boyne Hollow Chert (Bristow et al., 1995). Only the Boyne Hollow Chert occurs at outcrop in the Ringwood district. The formation ranges from the Upper Albian inflatum Zone (varicosum Subzone) to the dispar Zone (perinflatum Subzone).

The Boyne Hollow Chert Member (BHC) comprises up to 15 m of glauconitic sand and weakly cemented glauconitic sandstone with cherty and siliceous concretions and beds of chert (up to 0.6 m thick) in places. The base and the top of the member are taken, respectively, below the lowest, and above the highest chert bed or nodule horizon. The chert beds were formerly quarried for road metal, but there are few permanent exposures in the district. Grey, brown and black chert is widespread as field brash near Berwick St John [ST 9450 2260] and around [ST 9450 2195] and there are small exposures of chert in Blind Lane [ST 9436 2181] and Luke Street [ST 9461 2228]. A temporary cutting at Bowerchalke [SU 0200 2300] exposed 0.2 m of black to brown chert, above 2 to 3 m of green, glauconitic, weakly cemented sandstone containing shell detritus.

Chalk Group (Ck)

The chalk accumulated on the outer shelf of an epicontinental, subtropical sea at a time of relatively high sea level and in the absence of a supply of land-derived detritus. It is a white to greyish white, very fine-grained, microporous limestone composed predominantly of coccoliths (microscopic, calcareous skeletal remains of planktonic algae). Coarse-grained carbonate material is also present and includes foraminifera, ostracoda, and entire or finely broken echinoderm, bryozoan, coral, inoceramid and other bivalve remains. The chalk sequence can be subdivided (Figure 3) on its lithological variations, which include:

• the presence or absence of flint, a syndepositional or late stage diagenetic product
• chalkstone, in the form of mineralised hardgrounds that indicate periods of local shallowing and erosion
• calcarenites and 'marls', some are volcanogenic in origin and represent 'time-planes' within the succession

Calcareous mudstone ('marl') and argillaceous or muddy limestone occur in the basal part of the Chalk succession and elsewhere as individual thin seams. Mud-grade material forms 30 to 40 per cent of the basal strata but typically forms less than 5 per cent of the pure white chalk that dominates the upper part of the succession (Destombes and Shephard Thorn, 1971). The Fordingbridge and Cranborne boreholes prove a total thickness of 402 m and over 439 m, respectively, for the Chalk Group.

The lithostratigraphy of the chalk defined by Mortimore (1983, 1986) and Bristow et al. (1995, 1997) was subsequently refined and unified into a national scheme by Rawson et al. (2001; (Figure 3)). Using a combination of feature mapping, lithology, macropalaeontology, micropalaeontology and aerial photography (including Landsat imagery), the Chalk is divided into eleven mappable units within nine formations, all of which are laterally persistent. The thickness of the units is shown on the inside front cover (Geological succession).

The formations and members of the Chalk Group have very characteristic signatures on geophysical logs, especially the sonic log, and can be recognized across the whole of the Wessex Basin. The principal subdivision into two subgroups is taken at the base of a prominent but thin sequence of calcareous mudstones, the Plenus Marls Member that is easily recognised in the field and on gamma-ray geophysical logs. Below the Plenus Marls, the Grey Chalk Subgroup (GyCk) comprises thin argillaceous limestone and marly grey and greyish white chalk. The overlying White Chalk Subgroup (WhCk) comprises thick units of white and off-white nodular chalk with an upward increase in the proportion of flint nodules and beds. The traditional terms Lower, Middle and Upper Chalk are now only used informally.

Grey Chalk Subgroup (GyCk)

Where fully developed, the Grey Chalk Subgroup is divided into two formations, the lower is the West Melbury Marly Chalk Formation and the higher is the Zig Zag Chalk Formation. The subgroup constitutes the greater part of the traditional 'Lower Chalk'. To the south and west of this district the West Melbury Marly Chalk is missing over the Mid-Dorset Swell (Drummond, 1970) and part of south Dorset. Total thickness of the subgroup varies between 40 to 77 m.

West Melbury Marly Chalk Formation (WMCk)

This formation comprises grey or off-white, soft marly chalk, with regular thin firm or hard beds of chalk (limestone); it forms gently sloping ground. The basal part is glauconitic, sandy and conglomeratic, and it is separated as the Melbury Sandstone Member (it was formerly included in the Upper Greensand by Bristow et al., 1995). The top is taken at the base of the 'Cast Bed', a distinctive fine-grained calcarenite that marks the mid-Cenomanian erosional break, or in full successions where the erosional break is absent it is taken at the top of the feature that forms the Tenuis Limestone, a pale greyish brown, rough textured, calcarenitic limestone with Schloenbachia. The limestone is distinguished by the presence of the inoceramid bivalve I. tenuis and by its uneven hackly fracture, particularly after frost action.

The Melbury Sandstone Member (MelS) is exposed in small cuttings along the road between Berwick St John [ST 9500 2233] and Alvediston [ST 9765 2342], and in the road around Alvediston [ST 9703 2306] to [ST 9781 2356] where there are up to 0.5 m of medium- to coarse-grained glauconitic sand, weakly cemented in part, with scattered shell fragments. In the Shaftesbury district to the west, exposures show fine-grained glauconitic sandstone with siliceous and phosphatic concretions that include a rich fauna of ammonites, echinoids and brachiopods, which indicates an early Cenomanian age (Bristow et al., 1995). The exact thickness of the Melbury Sandstone Member is uncertain and it may be absent in the south of the district.

Exposures of West Melbury Marly Chalk Formation are restricted to the north-west of the district and include small sections along sunken tracks e.g. [ST 9545 2200] east of Berwick St John, where the beds consist of off-white, cream, pale yellow-brown and grey marl.

Zig Zag Chalk Formation (ZCk)

Typically, the Zig Zag Chalk consists of firm to medium hard, greyish white, blocky chalk. The base is taken at the 'Cast Bed', which is associated with a marked break in slope. The formation includes thick beds of harder chalk in contrast to the more marly West Melbury Marly Chalk; the formation forms a narrow outcrop at the foot of the Chalk escarpment in the north-west of the district. The Zig Zag Chalk is about 36 m and 40 m thick in the Cranborne and Woodlands boreholes, respectively, and about 30 m thick at outcrop. The formation increases in thickness eastward.

Small exposures of Zig Zag Chalk at Precombe Cottage [ST 9988 2460] and Ebbesbourn Wake [ST 9945 2430] comprise grey to cream, soft marly and blocky chalk. A cutting at Woodminton Farm [SU 0065 2243] exposes 10 m of buff-grey, medium to hard chalk with numerous marl seams.

White Chalk Subgroup (WCk)

The White Chalk Subgroup comprises seven formations. The lowest two form a distinctive unit, formerly known as the Middle Chalk, which comprises the Holywell Nodular Chalk and the New Pit Chalk formations. The Holywell Nodular Chalk Formation includes the Melbourn Rock, a hard and nodular bed up to 3 m thick. The New Pit Chalk Formation is a massively bedded chalk, generally with conspicuous marl seams. The uppermost five formations are predominantly white flinty chalk. They are the Lewes Nodular, Seaford, Newhaven, Culver and Portsdown chalk formations, and are approximately equivalent to the former Upper Chalk. The combined thickness of the seven formations is 366 and 372 m in the Woodlands and Cranborne boreholes, respectively.

The upper part of the White Chalk thins markedly to the north and north-east of the district. The Culver and Portsdown chalks are together about 175 m thick in the south of the district around Horton [SU 030 075], and also in the Woodlands and Cranborne boreholes, thinning north-eastwards to about 90 m thick in the Pentridge area [SU 034 178] and about 60 m between Rockbourne [SU 114 182] and Breamore [SU 151 186].

Holywell Nodular Chalk Formation (HCk)

This formation is the most readily recognizable chalk unit in the north-west of the district. The Plenus Marls Member at the base is overlain by hard chalk of the Melbourn Rock Member, which passes up into flaggy, shell-detrital chalk with conspicuous pink shell fragments of M. mytiloides. The Plenus Marls comprise alternating brightly coloured, calcareous mudstone (marl) and chalk or chalky marl, typically 2.5 to 4.5 m thick. In borehole geophysical logs, they give rise to a strong gamma-ray signal, which makes them readily recognisable. The overlying Melbourn Rock consists of up to 5 m of hard, nodular, poorly fossiliferous chalk that produces a characteristic surface brash and a strongly developed positive feature. The higher beds are nodular and calcarenitic, containing abundant pink shells and fragments of the bivalve Mytiloides. Interpretations of geophysical logs suggest that the formation ranges from about 22 to 25 m in the Cranborne and Woodlands boreholes, and may be as much as 30 m thick in the north-west of the district.

The Plenus Marls are rarely exposed but can be identified in field brash and along a track [ST 9665 2223] leading towards Ox Drove where a cutting exposes 10 cm of grey to green calcareous mudstone. Hard nodular chalk of the Melbourn Rock (about 5 m thick), but lacking significant shell debris, overlies the mudstone. The overlying strata comprise hard to very hard, nodular chalk with flaser marls throughout. The beds are commonly shelly and with a gritty texture. Numerous small cuttings [ST 9975 2431], [ST 9756 2199], [ST 9880 2460] and [SU 0513 2441] expose nodular chalk with abundant pink shells of Mytiloides, in sections 2 to 3 m thick.

New Pit Chalk Formation (NPCk)

This formation comprises blocky fractured, firm to medium hard, smooth, white chalk with regularly spaced pairs or groups of marls, each up to 15 cm thick. Flints occur in significant quantities in the highest parts of the formation. Typically the formation forms a slight negative feature between hard nodular chalks that lie above and below it. The base is taken where the nodular and shell-detrital chalk dies out, and it is mapped on the presence or absence of shell-detrital chalk brash. The New Pit Chalk is 18 m thick in the Cranborne Borehole, and between 12 and 20 m thick in the north-west of the district.

There are few exposures of New Pit Chalk in the district. A section [ST 9484 2474] in the track south-east of Hornwood Dairy Farm shows 2 m of rubbly, firm, off-white chalk with scattered Inoceramus fragments. The beds contain grey flints (10 to 20 mm across) and larger knobbly flints (up to 200 mm) with the paired valves of Inoceramus lamarcki geinitzi Tröger.

Lewes Nodular Chalk Formation (LeCk)

Significant numbers of nodules and flints within this formation mark the beginning of the flinty chalk within the Chalk Group. The Lewis Nodular Chalk Formation comprises hard to very hard, nodular chalk with interbeds of soft to medium hard chalk and marl, and characteristic large nodular flint seams that are consistent and regularly spaced.

The formation forms a positive feature at the top of the Chalk escarpment and typically caps the hills at the escarpment edge (Figure 4). It can be traced from Ox Drove [ST 955 207] in the west to Knowle Hill [SU 035 231] and Faulston Down in the east.

The base of the Lewes Nodular Chalk is taken at the lowest identifiable hard nodular chalk, which is equated with the 'Spurious Chalk Rock' of the Shaftesbury district (Bristow et al., 1995). The lowest strata consist of a persistent marl (generally considered to be one of the Glynde Marls of Mortimore, 1986) overlain by a conspicuous and feature-forming unit of hard to porcellanous chalkstone, about 1 to 5 m thick, containing erosion surfaces or hardgrounds that are glauconitic and phosphatic. The formation base is distinguished easily in the field by its characteristic brash, which is hard, dirty, hackly and fractured, and associated with flints; in boreholes it can be recognised across the whole of southern England by the characteristic signature on sonic logs. Nodular and hard beds become more widely spaced upwards within the Lewes Nodular Chalk, and there is a transitional zone of a metre or two with the overlying formation that may be picked out by a 'belt' of carious flints.

The Lewes Nodular Chalk ranges from 20 to 34 m thick within the district, and thickens towards the north and east with up to 50 m in the Salisbury (Farrant et al., 2000) and Winchester (Booth, 2002) districts.

An old pit [ST 9486 2092] at the top of Eastern Hollow, to the south of Berwick St John exposes 3.7 m of firm, white nodular chalk with flints. A pebble bed with scattered green-coated clasts up to 50 mm across occurs toward the top of the exposure. Flint nodules at the locality contain the echinoids Micraster and Echinocorys, the brachiopods, Cretirhynchia cuneiformis and Gibbithyris subrotunda and the bivalve Spondylus spinosus.

Seaford Chalk Formation (SCk)

This formation is composed of fine-grained soft white chalk and contains seams of large nodular and semitabular flints. It is free of marls, except at the base, and thin beds of hard nodular chalk together with abundant shell detritus are also present in the lower beds, associated with seams of carious flints. Unlike most other chalk units, there is rarely a consistently developed topographical expression at the base. The Seaford Chalk forms long dip slopes of the primary chalk escarpment (Figure 4).

Typical brash from the lower part of the Seaford Chalk contains abundant fragments of the bivalves Volviceramus and Platyceramus (Woods, 1997). Where these bivalves are absent, the brash is distinguished by the flaggy bedding and pure white of the soft chalk. Higher in the sequence, the flints are black and bluish black, mottled grey and with a thin white cortex; they occur with shell fragments of the brachiopod Gibbithyris and also of the bivalves Cladoceramus, Pycnodonte and Cordiceramus (Woods, 1997).

Permanent exposures of Seaford Chalk are sparse. In the north of the district, a cutting [SU 1632 2264] just south of The Giant's Chair exposes up to 6 m of soft, blocky white chalk with tabular flint, Platyceramus and the echinoid Conulus. Nearby, a pit [SU 1654 2396] exposes 8 m of soft to firm flinty chalk with Platyceramus and Volviceramus. A temporary pit at Higher Bridmore farm [ST 9640 2077] yielded Plagiostoma, Platyceramus, and Volviceramus involutus. Field brash near Martin [SU 0702 1993] consists of firm to soft tabular chalk containing many large flints and fragments of thick-shelled bivalves such as Platyceramus, Cladoceramus and Volviceramus. The flints are typically black to bluish black, and mottled grey with a thin white cortex and commonly contain shell fragments.

Newhaven Chalk Formation (NCk)

This succession is composed of soft to medium-hard, smooth white chalk with numerous marl seams and regular but sparse bands of flint. Typically, the marls are 20 to 70 mm thick; locally they become thinner or die out over the tectonic highs and are better developed and more numerous in the basins, for example around Salisbury (Mortimore, 1983, 1986).

Field brash of the Newhaven Chalk comprises smooth, angular, flaggy fragments of white chalk and tends to be much more abundant and with fewer large flints than the Seaford Chalk. The appearance of abundant flints with Zoophycos (a spiral trace fossil) and a 'meso-fauna' of oysters and crinoid debris near the base of the formation serve as useful markers for mapping the lower boundary. Individual thecal plates of the zonal index crinoid,

Marsupites testudinarius, also occur in the lower half of the formation, and can be seen in numerous small pits, track exposures and as brash; otherwise macrofossils are rare. The higher part of the formation equates with much of the echinoid Offaster pilula Zone.

The Newhaven Chalk is exposed in a large roadside quarry at Barford Cliff [SU 1909 2282], Downton, where up to 6 m of blocky white chalk with marl seams yield a diverse macrofossil fauna that includes various forms of the echinoid Echinocorys indicating the middle of the formation. A roadside pit north-east of Barford farm, [SU 1875 2282] contains the zonal crinoid U. socialis. The formation is best seen in the Brickworth Down landfill site [SU 216 243], about 1 km north of the district, where up to 30 m of soft white chalk with marl seams and the zonal index, Offaster pilula are exposed. West of the Avon, at Tidpit [SU 079 190] on Windmill Hill, the fauna includes forms of Echinocorys that indicate the higher part of the formation.

Culver Chalk Formation (CCk)

This formation comprises uniform, firm, white, flinty chalk. Over much of its outcrop, the Culver Chalk is divided into the Tarrant Chalk Member and overlying Spetisbury Chalk Member. The base of both members is defined at the crest of prominent scarp features (Figure 3); (Figure 4). The formation is up to 135 m thick in the south of the district but decreases north-eastward to a minimum of 40 m between Rockbourne and Downton.

The type area of the Tarrant Chalk Member (TCk) is the Tarrant valley, in the south-west of the district. The member consists of firm white chalk without significant marl seams, but with large, relatively widely spaced, nodular and semitabular flint bands (Bristow, 1991). The base is taken just below the maximum break of slope at the top of the secondary scarp ((Figure 4), Feature 1); the higher beds form the dip slope to the south and west. This scarp is well seen at Launceston Down [ST 951 104] where it extends eastward beyond the district. East of the Tarrant valley, the scarp is less pronounced, being partially masked by two higher scarp features (Features 2 and 3). The Tarrant Chalk is about 60 m thick in the Woodlands Borehole and thins to about 10 m in the north of the district. There is evidence to suggest that a period of synsedimentary channelling of the sea bed occurred at this time (Evans and Hopson, 2000), brought about by movement on the major faults within the basin and this may account for the substantial reduction in thickness.

Exposures of Tarrant Chalk in the north of the district include a pit located on the west side of the road just north of Down House, Redlynch [SU 1974 2192], which yields the belemnite Gonioteuthis and other species indicative of the highest part of the quadrata Zone. Another large pit [SU 1620 2007] on the west bank of the Avon at North Charford Drove, exposes up to 7 m of firm, blocky, flinty chalk with Echinocorys scutata. Typical field brash near Knoll Down [SU 0941 1869] consists of soft white blocky chalk with marl seams, and some bryozoan-rich and iron-stained spongiferous chalks.

The type-section occurs in a large roadside pit [ST 9427 0666] (Barton, 1992) below the cliff at Tarrant Rawston in the Shaftesbury district (Sheet 313) to the west. The Spetisbury Chalk Member (SpCk) consists of firm, white chalk with large flints, including tabular varieties in the lower part and Zoophycos flints in the higher part. The Spetisbury Chalk has a characteristic sonic velocity signature on downhole logs, which suggests that it is capped by a hardground or prominent flint layer. It forms well featured ground associated with a scarp ((Figure 4) Feature 2), which can be seen in the Blagdon Hill area [SU 056 182].

The thickness of the Spetisbury Chalk in the Wessex Basin is typically in the range 40 to 60 m and may be as much as 70 m in the Woodlands Borehole. The member thins northward across Cranborne Chase with 10 m or less at the northern margin of the Cranborne–Fordingbridge High around Rockbourne and Downton.

A pit [SU 1613 2010] on the west bank of the Avon, east-south-east of North Charford Down Farm, exposes 12 m of pinkish stained flinty chalk with an extensive fauna that includes bryozoan, Gonioteuthis and Echinocorys. In field brash near Whitsbury [SU 1330 1945] the Spetisbury Chalk consists typically of soft, white, blocky chalk with flint seams. The flints are of two types: high angled sheet flints and complex overgrown Zoophycos flints with lilac-coloured skins.

There are long dip slopes of Spetisbury Chalk, typically covered with clay-with-flints, above The Cliff, west of Witchampton [ST 989 065]. Fauna recovered from this area includes Echinocorys brydonei and Bourgueti­ crinus elegans and Echinocorys marginata indicative of the quadrata Zone.

Portsdown Chalk Formation (PCk)

This is the youngest of the chalk formations preserved in the district, and consists of soft to firm white chalk with common marl seams and some flints; in its lower part there are several horizons rich in inoceramid shell debris. The general scarcity of flints, together with common Belemnitella, the zonal index belemnite, is diagnostic. The formation base is taken at a negative feature at the base of a third element ((Figure 4), Feature 3) of the secondary escarpment. The scarp is well developed from Bradford Barrow [ST 981 047] to Blagdon Hill [SU 056 182] to Whitsbury [SU 127 193] and is less pronounced farther north and east. South of Wimborne St Giles, in St Gile's Park [SU 039 112], around Horton and north-west of Chalbury [SU 025 080], there is a wide crop of Portsdown Chalk with common Belemnitella. The Portsdown Chalk is about 45 m thick in the Woodlands Borehole, and 20 m or less in the north of the district at Redlynch.

Exposures east of the River Avon include several pits north-west of Redlynch. The pit [SU 2120 2156] north of Lower Pensworth Farm exposes 6 m of soft chalk with impersistant marl seams, sparse flints and an extensive fauna including Belemnitella and Bourgueticrinus.

Palaeogene

The Palaeogene comprises a sequence of siliciclastic sand and clay that provide a marked contrast to the underlying chalk. The boundary is an unconformity where the highest Maastrichtian and much of the Lower and Middle Paleocene sequence seen elsewhere in the region is absent due to erosion. The lowest part of the Palaeogene sequence, the Reading Formation, is assigned to the Paleocene, and the remaining strata are assigned to the Eocene (Figure 5). The total thickness ranges up to about 300 m.

Lambeth Group

The basal Palaeogene strata are attributed to the Reading Formation in the Ringwood district. Elsewhere, for example in the London Basin, the Lambeth Group is divided into the Upnor and Reading formations (Ellison et al., 1994).

Reading Formation (Rea)

This formation thins westward and is absent in the south-west near Chalbury [SU 018 069]. In the Bournemouth district to the south, it is overlapped by younger beds of the London Clay Formation. The formation can be correlated between the Woodlands, Fordingbridge Gasworks, Fordingbridge 1 and Blashford boreholes (Figure 6). The basal bed of the Reading Formation (formerly known as the Reading Formation Basement Bed) is typically a greenish grey and greyish green glauconitic clayey sand; it is 3 m thick in Fordingbridge Gasworks Borehole where oyster shells are recorded. It indicates a short-lived brackish or marine incursion from the south, and probably corresponds to the Upnor Formation. The overlying strata comprise mainly red, yellow, dark brown and purple mottled clay with some clean sand. The variegated colour is attributed to soil formation on a low-lying coastal plain (Buurman, 1980). The clay commonly contains calcareous nodules, lenticular sand bodies and cross-bedded channel fills composed of well rounded flint gravel as seen in a pit [SU 211 212] north-east of Redlynch and at Castle Hill Wood (Plate 1).

The Reading Formation is 16 m thick in the Fordingbridge Gasworks Borehole, 24 m thick in the Verwood Rectory Borehole, 10 m thick in the Woodlands Borehole, 12 m in the Blashford Borehole and 10 m in Fordingbridge 1 Borehole (Figure 6). It is dated as Late Paleocene in age, based on the recovery of sparse dinoflagellate assemblages indicative of the hyperacanta Zone (Curry et al., 1978).

Thames Group

In boreholes, the Thames Group can be subdivided into the Harwich Formation, which is overlain by the London Clay Formation. The Harwich Formation is thin; it has not been differentiated at outcrop but has been included within the mapped London Clay. In the Woodlands, Fordingbridge Gasworks and Blashford boreholes, 3 to 5 m of lignitic sand and ferruginous or calcareous sandstone rest sharply on the colour-mottled clay of the Reading Formation and are overlain by the basal pebble bed of the London Clay. These deposits have been described by Reid (1902), Curry et al. (1978) and by King (1981).

London Clay Formation (LC)

In adjacent districts, Bournemouth to the south (Bristow et al., 1991) and Southampton to the east (Edwards and Freshney, 1987), the London Clay Formation has been divided on the occurrence of named sand members. The two schemes are different, reflecting the changes in basin morphology and depositional environment. To the east, fully marine conditions predominated, and the principal deposit was silty clay with sand and glauconitic sand bodies; these are interpreted as short-lived, nearshore or tidally influenced channel deposits. To the south, deposition was much more varied with a greater influence from nearshore, beach-barrier and tidal/fluvial environments, which introduced significant, laterally persistent, sand bodies into the marine sequence of silty sandy clay.

The London Clay Formation within the Ringwood district is intermediate in terms of depositional environment, and it is not certain whether the less continuous mapped sand units shown dividing the formation to the west of the River Avon can be correlated with the named members of the Bournemouth district to the south. In the north-east, the Whitecliff and Nursling Sand members are contiguous with those of the Southampton district.

The London Clay is about 115 m thick at maximum in the Fordingbridge Borehole (Figure 6) but may range down to 85 m towards the north-east. Significant sand bodies are common in the upper part of the sequence (see (Figure 6)) and it is difficult to map the top of the formation. Interpretation of the boreholes indicates that the lower part of the formation is principally clay and sandy clay, with ironstones, carbonate concretions and pebble beds occurring at several levels, and the whole can be divided into two crude coarsening-upward cycles. The uppermost 60 m or so of the formation is of intercalated sand and carbonaceous clay with the fine- to very fine-grained sand units ranging up to 15 m.

In the north-east the Nursling Sand Member consists of bioturbated, olive-grey, silty very fine-grained sand and clayey, silty sand with a few thicker beds of olive-grey clay. All contain scattered glauconite grains, and the whole member can be up to 20 m thick. The Whitecliff Sand Member consists of up to 21 m of greenish grey slightly clayey, sparsely glauconitic fine- to very fine-grained sand with traces of lignite.

Bracklesham Group (BrB)

The Bracklesham Group is a unit of late Early and Middle Eocene age (Curry et al., 1978). The group encompasses the strata between the top of the London Clay and the base of the Barton Group, and consists of interbedded sand and clay units each of which varies laterally in detail depending upon its location of deposition within a fluctuating fluviatile, lagoonal, beach barrier or shallow marine environment. Therefore, it follows that correlation between the successions in the north and east (named from similar lithologies in the Southampton district; Sheet 315) with those in the south and west (named from lithologies mapped in the Bournemouth district; Sheet 329) are open to reinterpretation as more evidence becomes available. Indeed consolidation of the terminology in the future may well result in some of the terms becoming redundant as correlations become firmer or the principles of sequence stratigraphy are rigorously applied.

In the southern part of the district, the group comprises the Poole Formation and overlying Branksome Sand Formation. The higher Boscombe Sand Formation is described here within the Barton Group (Bristow et al., 1991); it feathers out towards the north and east, where it may be equivalent to the higher part of the Selsey Sand Formation of the Bracklesham Group. Farther north-east, the group comprises the Wittering, Marsh Farm, and Selsey Sand formations. In the Southampton district, the glauconitic Earnley Sand Formation is part of the Bracklesham Group and represents a fully marine incursion but it is absent here. The resurvey shows that the Wittering and Marsh Farm formations of the Southampton district pass west into the Poole Formation. Glauconite is not a significant component in either of the successions in this district. The whole sequence was probably deposited in a fluviatile environment with transgressions (up to five recognised, Plint, 1983) from the south-east, which introduced marginal-marine lagoonal clay and beach-barrier sand units (Plint, 1983; Bristow et al., 1991). The higher part of the Bracklesham Group, above an unconformity, is represented in the north-east by the Selsey Sand Formation, which passes south-west into either or both of the Branksome Sand Formation and Boscombe Sand Formation (part of the Barton Group described below). The most clearly defined boundary that can be used for correlation of borehole logs is that at the base of the Barton Clay (above the Boscombe Sand Formation); this is equivalent to the fifth transgressive surface of Plint (1983).

Poole Formation (Pool)

To the west of the Avon, the Pool Formation crops out south from Alderholt, and is partially concealed beneath extensive river terrace deposits. It consists predominantly of sand with thick interbeds of clay. East of the Avon, the formation passes into finely interbedded sand and clay of the Wittering Formation (Wtt) and Marsh Farm Formation (MrF). These formations are concealed beneath high-level terrace deposits south of Bohemia [SU 205 192] and the exact nature of the interdigitation cannot be determined.

In the Blashford Borehole, the base of the Poole Formation appears as an abrupt negative shift on the gamma-ray curve, demonstrating the sharp, transgressive (first transgressive surface of Plint, 1983) and probably erosional base to the formation. To the south of the district, the Poole Formation consists of an alternating sequence of sand and clay (Bristow et al., 1991, fig. 12, p.34) within which sand-clay couplets are referred to in ascending sequence as the Creekmoor, Oakdale, Broadstone and Parkstone sand or clay members. These members overlap progressively to the north such that only the upper and younger two pairs are found within the Ringwood district. The Broadstone Clay (BstC) and Parkstone Clay (PkC) members consist of pale, medium to dark grey and dark brown clay and silty clay with thin units of clayey sand. The organic content ranges from very sparse to highly carbonaceous or lignitic. Red-stained varieties also occur. Each clay member is underlain by a sand, namely the Broadstone Sand (BstS) and Parkstone Sand (PkS) members, which comprise fine- to medium-grained, cross-bedded, locally pebbly sands. The total thickness of the Poole Formation ranges from 25 to 40 m in this district.

Part of the Poole Formation is well exposed in Bluehaze Pit [SU 118 074], where the Parkstone Sand (around 5 m thick; (Plate 2)) is exposed below the Parkstone Clay (Plate 3). Three main lithologies are present in the quarry:

• moderately sorted, fine- to medium-grained, trough cross-bedded sand
• very fine to fine-grained sand with thin clay partings
• discontinuous mud lenses, generally less than 0.5 m in thickness

In the north-eastern part of the district, the Wittering and Marsh Farm formations are together 25 m thick and crop out in heathland around Pound Bottom. The base of the Wittering Formation is taken at the appearance of weakly bioturbated, thinly laminated clay or sand that rests on a variety of London Clay lithologies (Edwards and Freshney, 1987). In the past, the clay of the Wittering Formation was worked widely from small pits for the manufacture of bricks. The overlying Marsh Farm Formation consists of two main lithologies: variably carbonaceous laminated clay with laminae and thin beds of fine-grained to very fine-grained sand and silt, and fine-grained to locally coarse-grained, sparsely glauconitic sand with a variable proportion of clay beds and laminae. The sandy facies can be seen in the large pit at Pound Bottom [SU 220 177].

Branksome Sand Formation (BrkS)

This name was introduced by Bristow et al. (1991) for the combined Bournemouth Freshwater Beds and Bournemouth Marine Beds. The type area is the cliffs between Bournemouth Pier and Canford Cliffs where the formation is 70 m thick; it consists of cross-bedded, fine- to medium-grained sand, interbedded fine-grained sand and clay and carbonaceous mud with abundant plant material. At the base of the formation is the third transgressive surface of Plint (1983). In the Ringwood district, strata assigned to the formation are 8 m thick and comprise fine- to coarse-grained sand with laminated carbonaceous clay. East of the River Avon, the Branksome Sand Formation passes into the lower part of the Selsey Sand Formation.

In a temporary exposure at Bluehaze Pit [SU 122 072], the Branksome Sand was seen to rest with sharp contact on the Parkstone Clay (Plate 3). The basal 3 m of the formation comprise medium- to coarse-grained, slightly ferruginous, cross-bedded sand. In an abandoned sandpit 300 m north of Verwood, a 4 m-high exposure [SU 0910 0975] can be seen in the partially degraded face beneath Stephens Castle. Here the formation consists of 'typical' interbedded sand and clay that dip gently south-east. A brown, highly carbonaceous clay seam occurs toward the top of the cliff.

Selsey Sand Formation (Slsy)

Selsey Sand Formation (Slsy) In the north-east of the district, the Selsey Sand forms the uppermost formation of the Bracklesham Group; it is 35 to 45 m thick. It crops out extensively across heathland between Islands Thorns Inclosure [SU 219 146] and Hampton Ridge [SU 191 135] near Frogham, and extends south-west through Ringwood to pass laterally into the Branksome and Boscombe sand formations of the Bournemouth district. The Selsey Sand consists dominantly of green to greenish grey silty sand, silty clay, and sandy clayey silt, commonly glauconitic and bioturbated. A shelly marine fauna occurs at several levels.

Barton Group

The Barton Group is divided into a lower predominantly argillaceous part, the Barton Clay, and an upper arenaceous part (shown as the Barton Sand on the 1899 edition of the geological map), which is here subdivided into the Chama Sand and Becton Sand formations. In the south, a basal sand unit, the Boscombe Sand Formation, occurs between the Branksome Sand and Barton Clay formations (see below).

The presence of glauconite throughout much of the group indicates deposition in a marine environment that fluctuated between marine shelf, upper shore-face and upper beach zones. The group crops out in the south-east of the district within the New Forest and to the east of Ringwood.

Boscombe Sand Formation (BosS)

Around Bournemouth, the basal strata of the group comprise about 25 m of fine- to medium-grained, well-sorted sand. Bedding characteristics indicate a marine influence at the base, but estuarine conditions prevailed during deposition of the upper part (Bristow et al., 1991). The base of the formation is represented by the fourth transgressive surface of Plint (1983). The formation thins rapidly north of Bournemouth and may be represented only by a thin bed within the Ringwood district; it is not mapped separately. The underlying Branksome Sand together with the Boscombe Sand is equivalent to, and is mapped as, the Selsey Sand Formation in the north and east of the district.

Barton Clay Formation (BaC)

This unit consists mainly of greenish grey to olive-grey, glauconitic clay that may contain very fine-grained sand and shells, mainly bivalves and gastropods. The formation is concealed beneath terrace deposits on Fritham Plain [SU 224 134], and forms the clay ground across Ocknell Plain [SU 218 113] and Broomy Walk [SU 203 106]. The top of the succession is fine-grained sand with clear laminations and known informally as the Warren Hill Sand 'Member'.

The formation ranges from 45 to 65 m thick in the district. The only fossiliferous section in the uppermost strata occurs at the former Seymour's Brickyard [SU 178 058] east of Poulner (see Reid, 1902 for a faunal list). The predominantly molluscan fauna does not provide precise age control for the Barton Clay of the district, but elsewhere the formation has been shown to be of early Late Eocene age (Curry et al., 1978).

Chama Sand Formation (ChmS)

This formation has a gradational contact with the underlying Barton Clay; it becomes coarser upwards, passing from greenish grey, glauconitic sandy clay into clayey fine-grained sand. The formation is 8 to 12 m thick. At the coastal type section, the lower part may contain some calcareous cemented lenses of shells, including common Chama squamosa, from which the name of the formation is derived. Elsewhere, including this district, the formation yields no fossils probably as a result of decalcification.

Becton Sand Formation (BecS)

The Becton Sand comprises well-sorted yellow to buff fine- to very fine-grained sand. No shelly fossils have been found but burrows of Ophiomorpha are present. The formation has an extensive outcrop to either side of the A31 trunk road around Ridley Plain [SU 201 069] and Picket Hill [SU 182 057] where it is 35 to 40 m thick. The uppermost sands of the Barton Group record a phase of coastal progradation across the shelly, marine shelf deposits of the Barton Clay (Plint, 1984).

Solent Group

The Solent Group includes the Late Eocene Headon Formation and the Early Oligocene Bouldner Formation, which together represent the youngest Palaeogene deposits in the Hampshire–Dieppe Basin. The Bouldner Formation does not crop out in this district.

Headon Formation (He)

Only the lower 10 m of the Headon Formation is present at outcrop in the south-east of the district. The formation can be augered near Vereley Farm [SU 199 051] where green clay with associated carbonaceous clay and lignite rests on clean fine-grained sand of the Becton Sand Formation. The Headon Formation contains thick-walled freshwater bivalves. In the Bournemouth district to the south, the base of the deposit is a carbonaceous silt that rests on a palaeosol with rootlets at the top of the Becton Sand. The lithologies and the freshwater and brackish faunas suggest that the lower part of the formation was deposited in a freshwater lagoon behind a beach-barrier sand.

Quaternary

About 35 Ma is estimated to have elapsed between the deposition of the youngest Palaeogene strata and oldest Quaternary strata in this district. During this time younger Palaeogene and Neogene strata (some still preserved outside the district) were deposited across much of southern England, and subsequently removed following uplift as part of the general inversion of the Wessex Basin.

During the Pleistocene, sea levels rose and fell in response to the development of ice caps. At times of glacial maxima, a periglacial environment was established with much subaerial erosion by solifluction and by an extensive river system which had a much lower base level as the English Channel dried out. Three such glacial maxima affected southern England; the most severe was of Anglian age.

During the intervening warm stages, marine transgressions caused drowning of the lower courses of the river systems, principally the Solent River and its tributaries, and the breaching of the Straits of Dover and the Needles/Swanage cliffs. Beach and near-shore sediments were deposited along the margin of the English Channel.

Much of the higher chalk ground of Cranborne Chase, as well as the heathland of the New Forest are overlain by thin and patchily developed deposits of clay, sand and gravel. The unconsolidated or superficial deposits include a variety of periglacial, and fluvial sequences traditionally attributed to the Quaternary, although reliable age data are sparse and the oldest part of the succession may be late Pliocene in age.

Clay-with-flints

Clay-with-flints typically comprises about 1 to 3 m of structureless, dark brown, silty clay with angular flints. Clast-supported flint gravel with no clay matrix occurs locally. Although angular flint clasts are predominant, well-rounded flint, quartzite, subangular chert, sandstone (including 'sarsen' stones) and chalkstone have also been recorded. In sections, for example at the disused Tarrant Rushton airfield [ST 9463 0554], the base of the deposit is cryoturbated and comprises unstratified broken flints piped into extremely fractured chalk. Down-slope movement results in clay-with-flints that passes laterally into flinty head on slopes in the valley bottom.

Clay-with-flints is widely distributed on most subdivisions of the White Chalk, especially in the south of the district, and shows that deposition occurred after tilting and erosion. The age and origin are uncertain, but a Pleistocene age is considered likely (e.g. Hodgson et al., 1967), and the clay-with-flints are thought to have formed where enhanced solution of chalk occurred during periglacial conditions with other sediment derived from the Palaeogene sequences (Wilson et al., 1958; Bristow et al., 1995).

Head

Head is widespread, and consists largely of locally derived, poorly sorted silt, sand and gravel. It occurs in coombes and dry valleys on the chalk downs, adjacent to wide spreads of alluvium and above Palaeogene strata, particularly in clay valleys. In general, the minimum mapped thickness is 1 m, while 2 to 3 m is typical and up to 5 m not uncommon. While solifluction is probably the principal process involved in the production of all types of head, soil creep and slip must also play a part particularly on the steepest slopes. Such deposits can cover lower Pleistocene deposits and this demonstrates that it is a multiphase deposit generated periodically over a long time within the Quaternary.

Gravelly Head

Gravelly Head occurs on slopes below the eroding edges of high-level terrace deposits in the south-east of the district. It consists of unsorted flint gravel the composition of which closely reflects that of its parent body, but with the possible addition of illuviated clay and silt material.

River Terrace Deposits

River Terrace Deposits occur at a number of levels in the district, and are developed along most of the principal valleys and their tributaries. The deposits range in height from 0.5 to about 100 m above the present-day floodplain and represent the eroded remnants of formerly more extensive, gravel-rich deposits. In general, terrace deposits with low numbers are better preserved; they are up to 2 or 3 km wide along the River Avon, occurring near or adjacent to alluvium, and lie at heights that are not much above sea level. Older terraces are geographically more extensive and are found at distances up to 12 km from the present-day valley axis, and up to 100 m above the valley floor.

Up to ten terrace aggradations have been delimited on the map, based principally on their height above the present-day stream. The highest (Tenth) has a considerable height range and was considered by Kubala (1980) to represent up to five separate aggradation events.

Most terrace deposits comprise an upper layer or overburden of gravely sandy clay, typically 0.8 m thick, which overlies 1 to 2 m of a mixed sand and gravel layer with a gravel base (Plate 4). The mean grading of the terrace deposits, based on BGS boreholes in the Avon catchment is about 19 per cent fines, 25 per cent sand and 56 per cent gravel (Kubala, 1980). The mean gravel composition of the former catchment deposits is 91 per cent angular or subangular flint, 7 per cent quartz and 2 per cent ironstone. The sand fraction consists predominantly of quartz, and is either medium or medium and coarse grade with some fine grade. The average thickness of the deposits in this catchment is about 2.6 m, with a range of 1.5 to 10 m in parts of some terraces.

Low-level terrace deposits on the Avon overlie organic remains of Ipswichian and probable early Devensian age, and terrace deposits around Salisbury contain Palaeolithic artefacts (late Middle Pleistocene; for a review see Maddy et al., 2000). Together, this data suggest a range of terraces ages up to 1 500 000 years before present.

Alluvium, with peat

Alluvium, with peat in places, occurs along the River Avon, with minor spreads along the numerous small streams that drain both the Chalk and the clay of the New Forest. Alluvium typically consists of up to 2.5 m of poorly stratified brown silt and clay, commonly organic or peaty, overlying thin 'suballuvial' gravel.

Artificial deposits

There are few artificial deposits in this rural district. A large area of Worked Ground where sand and gravel has been extracted occurs in the Avon valley. The more extensive deposits are shown on the 1:50 000 map but minor occurrences have been shown only on the 1:10 000 scale maps.

Made Ground

Made Ground is shown where waste material (e.g. from excavations and cuttings) has been deposited on the ground surface. The most extensive areas of made ground are associated with the development of town centres, and major road and rail construction. Developments such as modern trading estates have been built on raised ground to lessen flood risk. The extent of made-ground was determined largely by observation during the survey and from information provided by site investigation data.

Worked Ground

Worked Ground is shown where natural materials have been removed from quarries and pits, road and rail cuttings and for general landscaping. Worked ground in the district is largely associated with the extraction of sand and gravel for aggregate, and road construction, former brickpits in clay deposits and chalk formerly worked for lime.

Infilled Ground

Infilled Ground is shown where the natural ground has been removed and the void partly or wholly backfilled; the composition of infill is generally not known in detail, but may include waste material from excavations and cuttings, household and other industrial wastes. Where quarries and pits have been filled and the ground restored and landscaped, built on or returned to agricultural use, there may be little surface indication of the extent of the backfilled area. In such cases, the boundaries of these sites are taken from archival sources such as local authority records and old topographic and geological maps.

Structure

The structure of the district is simple. A uniform low dip of about 2° (30 m/km) to the south-east is shown on seismic sections by the basal Chalk and Lower Greensand reflectors. These same seismic lines also demonstrate a general eastward inclination for the buried Jurassic strata of about 1° (8 to 14 m/km) but these rocks are disrupted by numerous east–west-trending normal faults with displacements typically less than 50 m. Few of these deeper faults have a surface expression as they relate to the extension tectonics that prevailed during and immediately after deposition of the Jurassic strata. However some were reactivated during the 'Alpine' (mid Miocene) compressional tectonic phase.

In the north-west of the district, the Ferne Park Fault shows a gently curved surface trace that extends from Bowerchalke, 11 km westward towards Ferne in the Shaftsbury district (Bristow et al., 1995). Seismic data shows this fault to be nearly vertical with a surface downthrow to the north of about 5 to 10 m increasing at depth to about 50 m at the level of the Inferior Oolite. In this area, a major subparallel fault to the north of the Ferne Park Fault can be identified at depth on seismic sections, but it does not affect strata younger than the Lower Greensand. This second fault can be seen to downthrow strata to the south by up to 50 m, and the two together define a graben-like structure. The Ferne Park Fault is thought to be antithetic, or 'branching', from this second major extensional fault and may have formed in response to reactivation of this earlier structure. Other smaller faults, including the Berwick St John Fault [ST 920 210] to [ST 950 222], which has a downthrow of 10 m to the south-east, are associated with this fault 'system'.

Closely associated with the Ferne Park Fault 'system' is the Bowerchalke Anticline; the axial surface trace is also orientated east–west just to the south of the fault. The anticline was first recognised by Reid (1902) who noted the variable dips in the Chalk around Bowerchalke [SU 0175 2325] itself. The fold is open and asymmetric with a 2° dip to the south on the southern limb, and dips up to 10° to the north on the northern limb. The anticline development is probably similar to that outlined for the Mere Fault and Wardour Monocline structural pair about 10 km to the north (Barton et al., 1998) where Palaeogene basin contraction reactivated and reversed the movement on concealed previously extensional faults.

Chalk strata in the north-east of the district form part of a gently southward dipping limb of the Dean Hill Anticline. This major east– west-trending periclinal fold has its major expression on the Salisbury and Winchester districts to the north (White, 1912; Williams-Mitchell, 1956). The western part of the anticline runs south-west to Witherington Down [SU 20 24], veering west along the northern boundary of the Ringwood district. The fold is offset en échelon from the Bowerchalke Anticline, but its form, with a steeper northern limb (dipping at up to 20° to the north) and shallow southern limb (2° to the south), suggests that it may well be the result of reactivation of the same buried fault structure.

Chapter 3 Applied geology

Hydrogeology

The Chalk is the most important aquifer in the district, although water is also extracted locally from the principal sand units within the Bracklesham Group. Boreholes in the Chalk aquifer typically show the best yields within valleys where the uppermost 50 m often contains productive fractures (Allen et al., 1997). These authors give transmissivity and storage coefficient data for the Cranborne Chase area, although pumping tests have been carried out at very few locations. Strong springs occur at the base of the Chalk at Bowerchalke, and chalk springs elsewhere are perennial.

Strong springs also occur at the base of the Palaeogene cover, the best known of which is Sagles Spring [SU 1259 1683], and its associated Roman Villa, south-east of Rockbourne; others include South Damerham [SU 1074 1490] in Ashford Water, and the Watercress Beds [SU 0630 1305] in the Crane valley, near Cranborne. There is a Chalybeate Spring [SU 0763 1083] in the London Clay farther downstream in the Crane valley, while east of the Avon wells, like the Abbots Well [SU 1786 1292] at Frogham, and spring lines are associated with the base of both the Poole and Selsey sand formations, and beneath high-level River Terrace Deposits. Historically, the narrow and easily defensible crossing points along the River Avon at Downton, Fordingbridge and Ringwood provided suitable sites for the original settlements with a ready supply of water, both from the river and from shallow wells.

Bulk minerals

The Ringwood district has a limited variety of mineral resources, and most are no longer exploited. Aggregate is the most important present-day resource, and River Terrace Deposits have been extensively quarried north of Ringwood as sources of sand and gravel. Most of the aggregate has come from quarries that extend for about 2.5 km along the east bank of the river near Blashford in the Fourth River Terrace (Plate 4). Farther east, the Eighth River Terrace around Gorley Common [SU 165 115] has been widely exploited in the past. There are large resources in high-level terraces in the New Forest where closely spaced boreholes show that the thickness ranges from 1.5 to 6 m. Building Sand was formerly dug from the Reading Formation, both east of Redlynch [SU 211 212], and from a pit [SU 1870 1995] near Downton that exposes up to 10 m of fine- to medium-grained, locally pebbly sand with thin seams of sandy clay.

Brick Clay has been worked from various Palaeogene clays in the past, for the manufacture of bricks, pipes and tiles. The brickworks [SU 1790 1990] south of Downton formerly used sandy clay from a nearby pit [SU 1875 1964] in the London Clay. There is an old brick pit in Broadstone Clay at Ebblake House [SU 108 077] and one farther east in Parkstone Clay [SU 114 075]. Commercial ball-clay deposits, which are present in the Poole Formation of the Wareham Basin, do not occur in the Ringwood district where the clay members are generally too sandy or iron-stained.

Lime and marl were formerly dug from many chalk pits in the west of the district during the 19th century. This was principally for spreading on fields deficient in calcium carbonate, such as those on Upper Greensand or Palaeogene. Some chalk was converted to quick-lime for the production of mortar. Where this activity became partly industrialised small kilns were often constructed within the quarries frequently relying on local sources of wood or charcoal derived from 'coppiced' woodland.

Geotechnical considerations

Ground conditions relevant to construction and development are summarised for the principal engineering geological units in (Figure 7). Some low-lying areas in the Avon valley are prone to flooding. The flood risk at Downton remains high even though extensive flood defence work has been carried out since the war. Floodplain management, using dry channels farther downstream to hold water on the floodplain, together with improvements to the river channel capacity, should eventually reduce the risk.

Solution-collapse hollows (dolines) in the district are typically shallow, roughly circular, depressions 20 to 50 m across and up to 3 m deep. Most occur where the basal Palaeogene strata or river terrace deposits overlie the Portsdown Chalk, and where the solid strata are gently inclined. Solution hollows [SU 2341 2389] west of Whiteparish, in Reading Formation and Portsdown Chalk, and just below Penbury Knoll [SU 0393 1723] are typical. Some solution hollows occur in head-filled valleys cut in the Chalk, including those at Chettle [ST 9540 1336], Wimborne St Giles [SU 0016 1472] and south of Pentridge [SU 0304 1664]; these must act as major soak-aways during periods of heavy rainfall.

Where permanent streams flow off the Palaeogene or Quaternary, swallow holes may develop. These are usually 2 to 5 m across and only some 3 m deep, and may show open fissures that take the surface water underground. In many cases these swallow holes form part of a larger solution hollow within which more than one hole can develop. These waters tend to resurge farther downstream where groundwater intersects the topography, thus surface drainage migrates within these dry-valleys or 'winterbournes' dependant on the seasonal variation of aquifer recharge locally.

The age of the solution hollows and swallow holes is uncertain. Sperling et al. (1977) provide evidence for continuous and recent collapse, and conclude that these features form as a result of intense and localised solution activity promoted by highly acidic conditions under heathland vegetation.

Certain clay-rich formations contain significant amounts of smectite that can undergo volume change with variation in moisture content. The clays absorb water and expand during wet periods and lose water during droughts. This 'swell-shrink' effect can result in ground heave that causes structural damage if foundations are inadequate. Vegetation, especially trees, is a major factor in determining moisture content of the clay. Some Palaeogene clays contain moderate levels of smectite and have been known to cause occasional ground heave subsidence.

Mass movement and slope stability are not usually a development constraint within the district although a large area of landslip occurs around Burwood [SU 060 140], north of Cranborne, where rotational slips of Reading Formation and basal London Clay Formation form outliers above the Portsdown Chalk. Landslipped ground is present adjacent to some high-level terrace deposits in the east of the district, and excavation by man has locally been responsible for destabilising the slopes. Sands of the Poole Formation are often unstable beneath high-level terraces with movement across the underlying London Clay.

Natural radon emissions

Radon is a natural radioactive gas produced by the radioactive decay of radium and uranium. Radon is found in small quantities in all rocks and soils, although the amount varies from place to place. Geology is the most important factor controlling the source and distribution of radon (Appleton and Ball, 1995). Relatively high levels of radon emissions are associated with particular types of bedrock and unconsolidated deposits. The Upper Greensand Formation has the highest radon potential in the Ringwood district. Slightly elevated radon potential is associated with ground where Head gravel deposits overlie the Upper Chalk Formation.

Radon that enters poorly ventilated enclosed spaces such as some basements, buildings, caves, mines, and tunnels may reach high concentrations in some circumstances. Inhalation of the radioactive decay products of radon gas increases the chance of developing lung cancer.

Radon Affected Areas have been designated by the National Radiological Protection Board (NRPB) where it is estimated that the radon concentration exceeds the Action level (200 Bq m⁻³) in 1 per cent or more of homes (Green et al., 2002).

Approximately 4 per cent of the Ringwood district has been identified by the NRPB as Radon Affected.

Radon protective measures may need to be installed in new dwellings (and extensions to existing ones) in areas where it is estimated that the radon concentration exceeds the Action Level in 3 per cent or more of homes (British Building Research Establishment Report, BR 211, 1999).

Conservation sites

The Ringwood district has numerous Sites of Special Scientific Interest (SSSIs) together with other national and international conservation designations. The Avon valley has a great range of habitats and downstream of Fordingbridge is well known for Atlantic salmon. The New Forest has an important protected woodland flora.

Information sources

Enquiries concerning geological data for the Ringwood district should be addressed to the Manager, National Geological Records Centre, BGS, Keyworth. Other geological information held by the British Geological Survey relevant to the district includes published maps, memoirs and reports, borehole records, fossils, rock samples, thin sections, hydrogeological data and photographs. Searches of indexes to some of the collections can be made on the Geoscience Data Index System available in BGS libraries or through the web site http://www.bgs. ac.uk. BGS catalogue of geological maps and books is available on request (addresses on back cover).

Maps

Geology

1:50 000 and 1:63 360

The district covered by Sheet 314 Ringwood was originally surveyed on a scale of 1:63 360 (one inch to one mile) by H W Bristow and J Trimmer, and published as an engraved and hand coloured Geological Survey Sheet 15 [Old Series] in 1856. The district was resurveyed at 1:10 560 scale (six inches to one mile) between 1896 and 1900, and published with Drift in 1902. This was largely the work of C Reid on the Palaeogene strata and F J Bennett on the Chalk, together with additions by E E L Dixon.

1:10 000

For the most recent revision of the 1:50 000 Series Sheet 314 Ringwood, the component 1:10 000 National Grid maps are listed below, together with the initials of the surveyors and dates of survey, and associated Technical Reports. Copies of these maps are available for public reference in the libraries of the British Geological Survey in Keyworth and Edinburgh. Copies are available for purchase from the BGS Sales Desk.

Digital geological map data

In addition to the printed publications noted above, many BGS maps are available in digital form, which allows the geological information to be used in GIS applications. These data must be licensed for use. Details are available from the Intellectual Property Rights Manager at BGS Keyworth. The main datasets are:

DiGMapGB-625 (1:625 000 scale)

DiGMapGB-250 (1:250 000 scale)

DiGMapGB-50 (1:50 000 scale)

DiGMapGB-10 (1:10 000)

The current availability of these can be checked on the BGS web site http://www.bgs.ac.uk/products/digitalimaps/dig-mapgb.html

Geophysical maps

1:1 500 000

Colour shaded relief gravity anomaly map of Britain, Ireland and adjacent areas, 1996

Colour shaded relief magnetic anomaly map of Britain, Ireland and adjacent areas, 1996

1:625 000

Aeromagnetic map of Great Britain (and Northern Ireland), South sheet, 1965 Bouguer anomaly map of the British Isles, Southern sheet, 1986

Bouguer gravity and aeromagnetic anomaly maps are also available at a scale 1:250 000.

Geochemistry maps

1:625 000

Methane, carbon dioxide and oil susceptibility, Great Britain (South Sheet) 1995

Radon potential based on solid geology, Great Britain (South Sheet) 1995

Distribution of areas with above the national average background concentrations of potentially harmful elements (As, Cd, Cu, Pb and Zn), Great Britain (South Sheet) 1995.

Hydrogeological maps

1:625 000

Sheet 1 (England and Wales), 1977

1:100 000

Groundwater Vulnerability Map, East Somerset and South-west Wiltshire (Sheet 43); produced by the Environment Agency

Groundwater Vulnerability Map, North-west Hampshire (Sheet 44); produced by the Environment Agency

Mineral maps

1:1 000 000

Industrial minerals resources map of Britain, 1996

Books and reports

British regional geology

Hampshire Basin and adjoining areas. Fourth edition. 1982

Memoirs

Geology of the country around Ringwood, (Sheet 314), 1902 This book is out of print: a facsimile copy may be purchased from BGS library at a tariff that is set to cover the cost of copying

Mineral Assessment Reports

Reports of the Institute of Geological Sciences describing the sand and gravel resources of parts of the district are available for the following 1:25 000 scale sheets:

SU 00, 10, 20, SZ 09, 19 and 29 — Bournemouth. No. 51

SU 11 and parts of SU 00, SU 01, SU 10, SU 20 and SU 21 — Fordingbridge. No. 50

Technical reports

BGS Technical Reports are not widely available, but may be consulted at BGS and other libraries or purchased from BGS.

Ten technical reports cover the geology of individual or combined 1:10 000 scale geological sheets; the reference numbers are given in the list above (pp.29–30) of 1:10 000 Series maps.

Robertson, A S. 1975. Record of wells in the area of new series one-inch (geological) Ringwood (314) sheet: draft copy. Institute of Geological Sciences, Well Catalogue Series, Water Supply Papers Sheet 314.

Biostratigraphy

There are 11 biostratigraphical reports covering the Ringwood district. These are held as internal open file reports and are available on application through the Library at BGS Keyworth. Enquiries about access to the palaeontological collections should be addressed to The Chief Curator, BGS, Keyworth.

Documentary collections

Boreholes and shafts

Borehole and shaft data for the district is catalogued in the BGS archives (National Geological Records Centre) at Keyworth on individual 1:10 000 scale sheets. For the Ringwood district, there are sites and logs for about 535 boreholes, for which index information has been digitised. For further information contact: The Manager, National Geological Records Centre, BGS, Keyworth.

Geophysics

Gravity and aeromagnetic data are held digitally in the National Gravity Databank and the National Gravity Aeromagnetic Databank at BGS Keyworth.

Hydrogeology

Wells and springs and water borehole records are held at the British Geological Survey, Hydrogeology Group, Maclean Building, Crowmarsh Gifford, Wallingford, Oxfordshire.

BGS Lexicon of named rock unit definitions

Definitions of the named rock units shown on the 1:50 000 Series Sheet 314 Ringwood are held in the Lexicon database. This is available on the Web Site http://www.bgs. ac.uk. Further information on the database can be obtained from the Lexicon Manager at BGS, Keyworth.

Material collections

Palaeontological collection

Macrofossils and micropalaeontological samples collected from the district are held at BGS Keyworth. Enquiries concerning all the macrofossil material should be directed to the Curator, Biostratigraphical Collections, BGS Keyworth.

Borehole core collection

The National Geosciences Records Centre, BGS, Keyworth, holds samples and entire core from a small number of boreholes.

BGS Photographs

Copies of the photographs used here are deposited for reference in the BGS Library, Keyworth. Colour or black and white prints and transparencies can be supplied at a fixed tariff.

Other relevant collections

Groundwater licensed abstractions, Catchment Management Plans and landfill sites

Information on licensed water abstraction sites for groundwater, springs and reservoirs, Catchment Management Plans with surface water-quality maps, details of aquifer protection policy and licensed landfill sites are held by the Environment Agency.

Earth science conservation sites

Information on the Sites of Special Scientific Interest (SSSI) within the Ringwood district is held by English Nature, Headquarters, Northminster House, Peterborough.

References

Copies of the references can be purchased from BGS Library, subject to the current copyright legislation; the BGS Library catalogue can be searched online at https://envirolib.apps.nerc.ac.uk/olibcgi

Allen, D J, Brewerton, L J, Coleby, L M, Gibbs, B R, Lewis, M A, MacDonald, A M, Wagstaff, S J, and Williams, A T. 1997. The physical properties of major aquifers in England and Wales. British Geological Survey Technical Report, WD/97/34. 312 pp. Environment Agency R & D Publication, 8.

Applton, J D, and Ball, T K. 1995. Radon and background radioactivity from natural sources: characteristics, extent and the relevance to planning and development in Great Britain. British Geological Survey Technical Report, WP/95/2.

Barton, C M. 1992. Geology of the Blandford Forum district (Dorset). British Geological Survey Technical Report, WA/91/81.

Barton, C M, Evans, D J, Bristow, C R, Freshney, E C, and Kirby, G A. 1998. Reactivation of relay ramps and structural evolution of the Mere Fault and Wardour Monocline, northern Wessex Basin. Geological Magazine, Vol. 135, 383–395.

Booth, K A. 2002. Geology of the Winchester district — a brief explanation of the geological map. Sheet Explanation of the British Geological Survey. 1:50 000 Sheet 299 Winchester (England and Wales).

Bristow, C R. 1991. Geology of the Tollard Royal — Tarrant Hinton district. British Geological Survey Technical Report, WA/91/20.

Bristow, C R, Freshney, E C, and Penn, I E. 1991. The geology of the country around Bournemouth. Memoir of the British Geological Survey, Sheet 329 (England and Wales).

Bristow, C R, Barton, C M, Freshney, E C, Wood, C J, Evans, D J, Cox, B M, Ivimey-Cook, H C, and Taylor, R T. 1995. Geology of the country around Shaftesbury. Memoir of the British Geological Survey, Sheet 313 (England and Wales).

Bristow, C R, Mortimore, R N, and Wood, C J.1997. Lithostratigraphy for mapping the Chalkof southern England. Proceedings of the Geologists'Association, Vol. 108, 293–315.

Buurman, P. 1980. Palaeosols in the Reading Beds (Palaeocene) of Alum Bay, Isle of Wight, U K. Sedimentology, Vol. 27, 593–606.

Chadwick, R A. 1986. Extension tectonics in the Wessex Basin, southern England. Journal of the Geological Society of London, Vol. 143, 465–488.

Curry, D, Adams, C G, Boulter, M C, Dilley, F C, Eames, F E, Funnell, B M, and Wells, M K. 1978. A correlation of Tertiary rocks in the British Isles. Geological Society of London Special Publication, No. 12, 1–72.

Destombes, J P, and Shephard-Thorn, E R. 1971. Geological results of the Channel Tunnel site investigation 1964–65. Report of the Institute of Geological Sciences, No. 11.

Drummond, P V O. 1970. The mid-Dorset Swell. Evidence of Albian–Cenomanian movements in Wessex. Proceedings of the Geologists' Association, Vol. 81, 679–714.

Edwards, R A, and Freshney, E C. 1987. Geology of the country around Southampton. Memoir of the British Geological Survey, Sheet 315 (England and Wales).

Ellison, R A, Knox, R W O, Jolley, D W, and King, C. 1994. A revision of the lithostratigraphical classification of the early Palaeogene strata of the London Basin and East Anglia. Proceedings of the Geologists' Association, Vol. 105, 187–197.

Evans, D J, and Hopson, P M. 2000. The seismic expression of synsedimentary channel features within the Chalk of southern England. Proceedings of the Geologists' Association, Vol. 111, 219–230.

Farrant, A R, Hopson, P M, Booth, K A, and Aldiss, D T. 2000. Geology of the Bourne River, Wiltshire. Final report on the geology of the Bourne and Nine Mile River catchments. Report of the British Geological Survey.

Green, B M R, Miles, J C H, Bradley, E J, and Rees, D M. 2002. Radon Atlas of England and Wales. National Radiological Protection Board Report NRPB-W26. (NRPB: Chilton, Didcot.)

Hamblin, R J O, Crosby, A, Balson, P S, Jones, S M, Chadwick, R A, Penn, I E, and Arthur, M J. 1992. United Kingdom offshore regional report: the geology of the English Channel. (London: H MS O for the British Geological Survey.)

Hodgson, J M, Catt, J A, and Weir, A H. 1967. The origin and development of clay-with-flints and associated soil horizons on the South Downs. Journal of Soil Science, Vol. 18, 85–102.

King, C. 1981. The stratigraphy of the London Clay and associated deposits. Tertiary Research, Special Paper, No. 6.

Kubala, M. 1980. The sand and gravel resources of the country around Fordingbridge, Hampshire. Description of 1:25 000 sheets S U 11 and parts of S U 00, S U 01, S U 10, S U 20 and S U 21. Mineral Assessment Report of the Institute of Geological Sciences, No. 50.

Maddy, D, Bridgland, D R, and Green, C P. 2000. Crustal uplift in southern England: evidence from the river terrace records. Geomorphology, Vol. 33, 167–181.

Melville, R V, and Freshney, E C. 1982. British regional geology: Hampshire Basin and adjoining areas. Fourth edition. (London: H MS O for Institute of Geological Sciences).

Mortimore, R N. 1983. The stratigraphy and sedimentation of the Turonian– Campanian in the Southern Province of England. Zitteliana, Vol. 10, 27–41.

Mortimore, R N. 1986. Stratigraphy of the Upper Cretaceous White Chalk of Sussex. Proceedings of the Geologists' Association, Vol. 97, 97–139.

Penn, I E, Chadwick, R A, Holloway, S, Roberts, G, Pharaoh, T C, Allsop, J M, Hulbert, A G, and BurnsI M. 1987. Principal features of the hydrocarbon prospectivity of the Wessex– Channel Basin, U K. 109–118 in Petroleum geology of north-west Europe. Brooks, J, and Glennie, K (editors). (London: Graham Trotman.)

Plint, A G. 1983. Facies, environments and sedimentary cycles in the middle Eocene, Bracklesham Formation of the Hampshire Basin — evidence for global sea-level changes. Sedimentology, Vol. 30, 625–653.

Plint, A G. 1984. A regressive coastal sequence from the Upper Eocene of Hampshire, southern England. Sedimentology, Vol. 31, 213–225.

Rawson, P F, Allen, P W, and Gale, A S. 2001. The Chalk Group — a revised lithostratigraphy. Geoscientist, Vol. 11, 21.

Reid, C. 1902. The geology of the country around Ringwood. Memoir of the Geological Survey of Great Britain, Sheet 314 (England and Wales).

Robertson, A S. 1975. Abstracts of records of wells in the area of new series one-inch (geological) Ringwood (314) Sheet (unpublished). Institute of Geological Sciences.

Sellwood, B W, and Scott, J. 1986. A geological map of the sub-Mesozoic floor beneath southern England. Proceedings of the Geologists' Association, Vol. 97, 81–85.

Sellwood, B W, Scott, J, and Lund, G. 1986. Mesozoic basin evoloution in Southern England. Proceedings of the Geologists' Association, Vol. 97, 259–289.

Simpson, I R, Gravestock, M, Ham, D, Leach, H, and Thompson, S D. 1989. Notes and cross-sections illustrating inversion tectonics in the Wessex Basin. 123–129 in Inversion Tectonics. Cooper, M A, and Williams, G D (editors). Geological Society Special Publication, No. 44.

Smith, N J P (compiler). 1985. Pre-Permian geology of the United Kingdom (South). Scale 1:1 000 000. 2 maps commemorating the 150th anniversary of the British Geological Survey. (Surrey: Cooke Hammond and Kell for the British Geological Survey on behalf of the Department of Energy.)

Sperling, C H B, Goudie, A S, Stoddart, D R, and Poole, G G. 1977. Dolines of the Dorset Chalklands and other areas in southern Britain. Transactions of the Institute of British Geographers, N S2, 205–223.

White, H J O. 1912. The geology of the country around Winchester and Stockbridge. Memoir of the Geological Survey of Great Britain, Sheet 299 (England and Wales).

Williams-Mitchell, E. 1956. The stratigraphy and structure of the Chalk of the Dean Hill Anticline, Wiltshire. Proceedings of the Geologists' Association, Vol. 67, 221–227.

Wilson, V, Welch, F B A, Robbie, J A, and Green, G W. 1958. Geology of the country around Bridport and Yeovil. Memoir of the British Geological Survey (Sheets 327 and 312, England and Wales).

Woods, M A. 1997. A biostratigraphical review of the Chalk Group of the Ringwood (314) district and adjoining areas. British Geological Survey Technical Report, WH/97/151R.

Index to the 1:50 000 Series maps of the British Geological Survey

The map below shows the sheet boundaries and numbers of the 1:50 000 Series geological maps. The maps are numbered in three sequences, covering England and Wales, Northern Ireland, and Scotland. The west and east halves of most Scottish 1:50 000 maps are published separately. Almost all BGS maps are available flat or folded and cased.

(Index map)

The area described in this sheet explanation is indicated by a solid block.

British geological maps can be obtained from sales desks in the Survey's principal offices, through the BGS London Information Office at the Natural History Museum Earth Galleries, and from BGS-approved stockists and agents.

Figure and plates

(Figure 1) Structure of the Wessex Basin. a The principal elements of the Wessex Basin b Cross-section along the line C–B

(Figure 2) Major subdivisions of the strata proved in deep boreholes within (Cranborne, Fordingbridge and Woodlands) and adjacent to the Ringwood district. Thickness given in metres.

(Figure 3) Correlation of the Chalk Group.

(Figure 4) Schematic east–west cross-section showing the relationship of lithostratigraphical units to ground features on Cranborne Chase.

(Figure 5) Palaeogene formations in the Ringwood and surrounding districts. Location of boreholes and line of section shown in Figure 6.

(Figure 6) Borehole correlation of basal Palaeogene sequences in the Ringwood district (for line of section see (Figure 5)).

Plates

(Plate 1) Pebble bed in the Reading Formation at Castle Hill Wood [SU 0606 1253]. (Lens cap approximately 6 cm) (GS 1249).

(Plate 2) Poole Formation sands (Parkstone Sand Member) at Blue Haze Pit [SU 118 074]. Auger 1.3 m (GS 1250).

(Plate 3) Parkstone Clay sharply overlain by Branksome Sand at Blue Haze Pit [SU 1193 0700] (GS 1251).

(Plate 4 )Fourth River Terrace deposits at Ibsley Gravel Pit [SU 149 086]. Auger 1.3 m (GS 1252).

(Front cover) Stephens Castle, Verwood [SU 0917 0967] (Photograph P M Hopson; GS 1248). One of numerous exposures of the Poole Formation showing interbedded laminated silty clay and fine- to medium-grained sand. The silty clay beds are lenticular and generally comprise 'ball clay' or kaolinite-rich clay.

(Rear cover)

(Geological succession) Summary of the geological succession at outcrop within the district.

(Index map) Index to the 1:50 000 Series maps of the British Geological Survey

Figures

(Figure 2) Major subdivisions of the strata proved in deep boreholes within (Cranborne, Fordingbridge and Woodlands) and adjacent to the Ringwood district

(Figure 7) Ground conditions associated with main geological units and their relevance to construction and development